Polyester resins, in particular poly(ethylene terephthalate) (hereinafter sometimes abbreviated as PET), which is produced from terephthalic acid and ethylene glycol as raw materials, are extensively used as many materials and products such as, e.g., fibers, textiles, molding resins, and beverage bottles.
In order for a polyester resin to have the moldability and mechanical properties which are necessary for those applications, it should have a degree of polymerization increased to a given level. A method in wide industrial use for this purpose comprises subjecting raw materials for a polyester to melt polycondensation to obtain a polycondensation product having a relatively high viscosity and subsequently subjecting the polycondensation product to solid-phase polycondensation. However, the solid-phase polycondensation in this method heretofore in use necessitates a relatively long time period. There is hence a desire for a production process attaining better productivity.
A process for polyester resin production has been proposed as a process with improved productivity. This process comprises subjecting raw material monomers for a polyester to melt polycondensation to obtain a melt polymerization polymer having a relatively low degree of polymerization and subjecting this melt polymerization polymer to solid-phase polycondensation. For example, JP-T-10-512608 (WO 96/22319) discloses a process in which a lowly polymerized melt polymerization polymer obtained by melt polycondensation and having an average degree of polymerization of from about 5 to about 35 (intrinsic viscosity of about from 0.10 to 0.36dL/g) is crystallized so as to result in a crystallite size of 9 nm or larger and then subjected to solid-phase polycondensation. (The term “JP-T” as used herein means a published Japanese translation of a PCT patent application.) There is a statement therein to the effect that according to this process, polycondensation can be initiated at a higher temperature, e.g., 230° C., preferably at 240° C., to directly obtain the target polymer. However, our investigations have revealed that a satisfactory rate of solid-phase polycondensation is not always obtained possibly because the degree of polymerization at the time when the solid-phase polycondensation is initiated is too low or because crystals grow to inhibit the movement of molecules, although the reasons are unclear.
On the other hand, U.S. Pat. No. 6,284,866 discloses a process for producing a copolyester for use as bottles reduced haze in low-temperature. In this process, a melt polymerization polymer which is a polyester in which the sum of a dicarboxylic acid component copolymerized (mol %) and a diol component copolymerized (mol %) is 6 or more and which has an intrinsic viscosity of from 0.25 to 0.40 dL/g is subjected to solid-phase polycondensation under specific conditions. However, the process disclosed in that patent document is not always industrially advantageous because the solid-phase polycondensation is conducted with a rotary-vacumn tumble dryer and is solid-phase polycondensation under high vacuum. Furthermore, our investigations revealed that the melt polymerization polymer has a relatively low melting point because of the too high proportion of comonomer units and, hence, a high temperature cannot be used for the solid-phase polycondensation when it is not conducted under high vacuum. Consequently, the rate of solid-phase polycondensation is low and the process is not efficient when it is not conducted under high vacuum.
An object of the invention is to provide a process in which the solid-phase polycondensation of a polyester is conducted at an exceedingly high rate and a polyester is hence produced with satisfactory productivity. Another object of the invention is to provide a process in which the solid-phase polycondensation of a polyester can be carried out at a high rate without the necessity of using special conditions, e.g., high vacuum, and a polyester is produced with satisfactory productivity without posing any problem concerning handling, such as, e.g., the fusion bonding of the resultant polyester resin to itself or the adhesion of the polyester resin to the reactor.